1. Field of the Invention
The present invention relates to the field of semiconductor assembly or packaging and, more particularly, to quad flat no-lead (QFN) packaging technologies.
2. Description of the Related Art
There are mainly two types of conventional lead frame structures used in semiconductor packaging processes. For the first type, as shown in FIG. 77, after performing chemical etching and electrical plating (or simply plating) on the metal substrate, a layer of high-temperature resistant film is affixed on the back surface of the metal substrate to form the lead frame carrier to be used in the packaging process.
For the second type, as shown in FIG. 79, chemical half etching is first performed on the back surface of the metal substrate, and the chemically half etched areas are sealed with an encapsulation material. Further, chemical half etching for inner leads is performed on the top surface of the metal substrate and followed by plating on the surface of inner leads of the lead frame so as to complete the lead frame.
However, both of these two types of lead frames may have certain disadvantages in the packaging process. For example, for the first type, the issues may include:                1) Because an expensive high-temperature film must be affixed on the back surface of the metal substrate, the manufacturing cost is directly increased;        2) In the die attaching process of the packaging process, also because a high-temperature resistant film must be affixed on the back surface of the metal substrate, only epoxy may be used in the die attaching process, and certain techniques such as the eutectic process or soft solder technique cannot be used, which greatly limits choices of available products;        3) In the wire bonding process of the packaging process, also because a high-temperature resistant film must be affixed on the back surface of the metal substrate and the high-temperature resistant film is a kind of soft material, the wire bonding parameters may become unstable, which seriously impacts on the quality of wire bonding and the reliability and stability of the product; and        4) In the molding process, also because a high-temperature resistant film must be affixed on the back surface of the metal substrate, the molding pressure during the molding process may cause certain mold bleeding between the lead frame and the high-temperature resistant film, which may change a conductive metal lead into an insulated lead, as shown in FIG. 78 (certain metal leads on the left side of the drawing are insulated by bleeding materials).        
For the second type conventional lead frame structure, the issues may include:                1) Because the etching process is carried out twice, manufacturing cost may be increased;        2) Because the composition of the lead frame is a metal material or compound, when operated in a high or low temperature environment, the lead frame may be warping due to the different expansion and shrinkage stress of the different materials;        3) The warpage of the lead frame may impact on the accuracy of die attaching process and may also affect the production yield due to the impact on the smooth transferring of the warping lead frames in the die attaching process;        4) The warpage of the lead frame may also impact the alignment accuracy of the wire bonding and may also affect the production yield due to the impact on the smooth transferring of the warping lead frames in the wire bonding process; and        5) Because the inner leads on the top surface of the lead frame are formed using etching technique, the width of the inner leads might have to be greater than 100 μm and the distance between two adjacent inner leads also must have to be greater than 100 μm. Thus, it may be difficult to achieve high density for the inner leads.        